Vibration table



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INVENTOR.

LYLE E MATTHEWS Irow/news July 17, 1962 E. MATTHEWS 3,044,292

VIBRATION TABLE Original Filed Dec. 17, 1958 2 Sheets-Sheet 2 INVENTOR.LYL E E MATTHEWS United States Patent O 3,044,292 VIBRATION TABLE LyleE. Matthews, Oxnard, Calif., assignor to the United States of America asrepresented by the Secretary of the Navy Original application Dec. 17,1958, Ser. No. 781,172. Divided and this application July 23, 1959, Ser.No.

1 Claim. (C1. 'ls-71.6)

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment `of any royalties thereon or therefor.

The present invention relates to apparatus by means of which aparticular vibratory environment may be simulated in order to determinethe performance of one or more components designed to operate in such anenvironment for an extended -period of time. The invention furtherrelates -to a driving mechanism, especially but not exclusively intendedfor use with such a vibration simulator, which acts to convert rotarymotion into reciprocating or `oscillatory movement. This -applicationconstitutes a `division of application Serial No. 781,172 filed December17, 1958.

It is frequently desirable to ascertain in advance the reliability of acomponent or assembly which will be subjected in actual use to severeand/ or prolonged shocks or vibrations. To accomplish this with lanydegree of accuracy, the apparatus employed for testing purposes must beable to simulate quite closely the actual conditions to be encounteredby lthe structure under investigation.

When the latter is of relatively small size and weight, the problem isnot too difficult of solution. However, especially as the weight factorincreases, conventional testing machines yield results which areprogressively less satisfactory. For example, practically all equipmentdesigned for shipboard use must withstand substantially continuousvibration (caused primarily by the ships propellers) while the vessel isunder way. Such vibration is usually of a fairly constant frequencybetween l5 and 2-0 cycles per second, with an amplitude up to one qdepending upon the cruising speed of the ship and the location thereon.Ordinarily, this is not too serious a matter, as most marineinstallations are of a rather massive nature. However, at -the presenttime various types of guided missiles are being mounted Ion aircraftcarriers or other vessels specifically designed to accommodate suchweapons, and, of course, the missiles, as Well as their associatedcheck-out equipment, must be ready for instant use. These missiles,especially in their guidance systems, incorporate a large number ofextremely minute parts which are `delicately lbalanced and criticallypositioned. Although of course these components are designed to be asrugged as possible, space limitations necessitate certain compromises inthis respect, and, in order to determine just what the limit is to whicha missile can be continuously vibrated before operational failureensues, preinstallation laboratory tests are a practical necessity.

In the example -above given, `a single missile and its associatedcheck-out apparatus may weigh in the neighborhood of 4,000 pounds andextend over a base area of as much as 100 square feet. A platform ofsuitable size and strength to support such la load may weigh 2,000pounds. Since a satisfactory laboratory vibrator should be capable ofproducing an acceleration of at least 2 gs, the required vibratory forcefor the missile assembly plus its platform is at least 12,000 pounds, or6 tons.v In addition, some 2,000 pounds of -this load may be on shockmounts, so that, at resonance, its acceleration is amplified 3,044,292Patented July 17, 1962 ice by a factor of three. This requires anadditional two tons of force to vibrate the platform. Thus, the testingdevice must be capable of developing from eight to ten tons of vibratoryforce. The situation is made even more complex by the fact that the loadis spread over a large surface area (the platform may be l0 x l0', forexample) and, in addition, the center of gravity of the load isfrequently offset from the center of the platform. Still further, alateral stabil-ity problem may arise if the loads center of gravity ismore than a few inches above the platform surface.

For optimum results, the vibrator should produce simple harmo-nic motionin the vertical direction with minimum angular movement about any axis.It should also operate equally well under all load conditions within itsrated capabilities. Finally, it should possess a minimum number ofadjustments for frequency, amplitude and balance, with such adjustmentsbeing readily accessible to the operator thereof.

A vibrator which satisfies the -above requirements is provided `by thepresent invention. In a preferred embodiment, it consists of a at tableor platform which is horizontally supported in spaced relation to arigid foundation through a plurality of toggle joints. Each toggle jointhas two arms, one of which is pivotally secured at one of its ends tothe table, and the other of which is pivotally secured at one of itsends to the foundation. The remaining ends of the arms are rotatablyattached to one another and to `a connecting rod which extendshorizontally in the space between the foundation and the lower surfaceof the platform. An oscillatory movement of this connecting rod resultsin a vertical displacement of the platform, first downwardly and thenupw-ardly. Three or four toggle joints are usually associated with eachconnecting rod, and 1a number of such connecting rod assemblies arrangedin parallel fashion so as to provide adequate support for all sectionsof the platform.

One object of the present invention, therefore, is to provide testapparatus for the laboratory simulation of a particular vibratoryenvironment.

Another object of the invention is to provide a socalled vibration tableadapted to produce vertical vibratory motion from a driving forceapplied in a direction generally transverse thereto.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. l is a plan view of a vibration simulator designed in accordancewith a preferred embodiment of the present invention:

FIG. 2 is an end view of the vibration simulator of FIG. 1 showingcertain of the toggle joint assemblies; and

FIG. 2a is an enlraged View of one of the toggle joint assemblies ofFIG. 2.

A number of techniques are presently known by means of which a load maybe vibrated. One of these employs the principle of resonance, and isembodied, for example, in a platform mounted on springs and driven by anoscillatory force. When the stiffness of the springs is such that thenatural frequency of the spring-load system is the same as that of thedriving frequency (or in other words, when the system is at reasonance)then the springs produce most of the vibratory force and very littleexternal power is required. A disadvantage is that large forces areapplied to the support on which the springs are mounted. Another methodutilizes the reaction thus generated by the rotation of unbalancedweights. This has the advantage over the resonant shaker of impartinggreatly decreased forces to the base or support on which the shaker ismounted.

Each of the above systems, however, has certain drawbacks. These include(l) a relatively high degree of instability, especially when the weightdistribution is nonuniform, (2) the mode of vibration varies when theload is at resonance, and (3) actual vibratory displacement is afunction of load, load distribution, and frequency.

To overcome the drawbacks of structures such as the above, the presentinvention provides for the development of linear vibratory motion fromoscillatory motion occurring in a plane normal to the vibrations. Thisis accomplished in the embodiment illustrated by means which includesthe table or platform of FIGS. l and 2. This table 10 may be fabricatedof some material such as one-inch thick aluminum alloy, and is of a size(10 x 10' is typical) dependent upon the particular dimensions of thepackage to be tested. Although not shown, a plurality of openings areformed in the table to accommodate a corresponding number of hold-downbolts, the location of such openings being again governed by thephysical characteristics of the object or assembly under investigation.Attached to the lower surface of table 10 to lend structural rigiditythereto are a plurality of IJbeams 12 also preferably formed of aluminumalloy and arranged in spaced-apart parallel fashion as shown in thedrawings.

The table assembly 10-12 is designed to be supported upon a solid base14 (such as a concrete foundation) in such fashion that it may undergolimited vertical displacement with respect thereto. For this purpose,there is provided a plurality of double toggle joints each of which isgenerally identified in the drawings by the reference numeral 16.Although the number of such toggle joint assemblies is obviouslydetermined by the surface dimensions of table 10, the drawingillustrates four rows of toggles parallel to and aligned with theI-beams 12, with `four toggles in each row. To eliminate orsubstantially reduce unloaded beam resonance in the first and secondnatural modes, each toggle joint assembly is located near the naturalmode nodal lines of the platform. The natural frequency of the thirdmode is beyond the operating limits of the illustrated device.

A representative toggle joint assembly is shown in FIG. 2a. It consistsof a pair of double toggle arms 18 and 20, the arm 18 being -pivotallyattached at its upper end to the support block 21. The latter is securedto the under surface of the I-beams 12 through the disc-shaped adaptor22, while the arm 20 is pivotally secured at its lower end to the baseor foundation 14 by means of the pillow block 23 and the adaptor 24. Theremaining extremities of the toggle arms 18 and 20 are pivotallyconnected by the pin 26 to form a knee joint, as illustrated.

Referring again to FIG. 2, the knee joints of the toggles making up asingle row are actuated by a common transverse drive rod 28, which, forexample, may be a horizontally-positioned bar of aluminum alloy havingopenings designed to receive the pins 26.

It should now be apparent that movement of the drive rod 28 essentiallyin a horizontal plane (alternately) from left to right in FIG. 2) willresult in a vertical displacement of the table 10. This displacementshould contain no appreciable horizontal component, and, consequently,four flexure members 30 are respectively attached to the corners of thetable 10 (as shown in FIGS. l and 2 only) to reduce any horizontalmotion of the table to a minimum. AIt is desirable to reverse thedirection of motion of the toggle knee joints in alternate rows toeectively cancel any horizontal thrust forces which might otherwise bedeveloped during operation.

Each transverse drive rod 2S has essentially simple harmonic motionimparted thereto by means of a connecting rod 34 (FIGS. 1 and 2) one endof which is pivotally connected to the drive rod 28 and the other end ofwhich is mounted through ball ybearings on an eccentric 36, the latterin turn being rotatably carried on a second eccentric 38. The innereccentric 38 is mounted on a drive shaft 40 for rotation therewith. Theouter eccentric 36 is adjustable in position with respect to the innereccentric 38, and, as a result of such an adjustment, the lateralmovement imparted to the member 28 (or, in other words, the throw of theconnecting rod 34) may be controllably varied from zero to maximum tocorrespondingly vary the vertical displacement of the platform or table10. In FIG. 2 the two eccentrics 36-38 are set for a maximum throw ofthe connecting rod 34.

As above mentioned, reversal of the direction of motion of the toggleknee joints in alternate rows is desirable in order to effectivelycancel any horizontal thrust forces which might otherwise be developedduring operation of the apparatus. This is accomplished by angularlyoffsetting the eccentric assemblies in alternate rows by For example,the mounting of the inner eccentric 38 of FIG. 2 on drive shaft -40 isangularly identical in rows 1 and 4 of FIG. l (reading down from thetop) and is off-set from this position by 180 in rows 2 and 3. Thus,there is a dilference in the direction of movement of the drive rods 28in alternate rows 1 and 3 as well as in alternate rows 2 and 4.

It will be noted that the essentially simple harmonic motion imparted tothe drive rod 28 produces a sinusoidal movement of the platform 10 attwice the oscillatory frequency of the drive rod. 'Ihisfrequency-doubling factor enables the drive shaft 40 to operate at arelatively low speed of rotation. For example, a drive shaft speed of600 r.p.m. develops a vibratory platform movement at the rate of 20c.p.s.

The shaft 40 is driven by a variable speed motor 42 (FIG. 1). It issupported by a plurality of roller bearing pillow blocks 44 one of whichis placed on each side of each connecting rod assembly.

`It might be expected that an olf-center relationship of the twoeccentrics 36-#38 as described above might result in a dynamic unbalanceof the crankshaft mechanism. Any such tendency, however, is readilyovercome by adding weights thereto. Specifically, the center of gravityof the outer eccentric (plus its bearing) and that part of theconnecting rod assembly directly associated therewith is adjusted to thecenter of the inner eccentric. Then weights are added to the innereccentric until the combined center of gravity of all of the rotatingparts is adjusted to the axis of the drive shaft. Such a techniqueprovides dynamic balance of yassembly regardless of the particulareccentricity setting selected,

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claim `the inventionmay be practiced otherwise than as specifically described.

I claim:

Apparatus for simulating a particular vibratory environment to determinethe response of an object thereto, said apparatus comprising ahorizontally-positioned table adapted to have said object securelymounted thereon, a stationary base member, means for supporting saidtable in spaced relationship to said base member so as to permit acyclic vertical displacement therebetween, a driving mechanism connectedto said supporting means, means for imparting a horizontal oscillatorymovement to said driving mechanism to bring about the cyclic verticaldisplacement of said table with respect to said base member, said meansfor supporting said table in spaced relationship to said base memberincluding a plurality of toggle joints, one end of each toggle jointbeing pivotally connected to said table, the other end of each togglejoint being pivotally connected to said base member, and the knee ofeach toggle joint being pivotally connected to said driving mechanism,said plurality of toggle joints being arranged in parallel rows, saiddriving mechanism including a plurality of horizontally-positioned driverods equal in number to the number of rows of toggle joints, one driverod being pivotally connected to the knees of each of the toggle jointsmaking up one of said rows, said means for imparting a horizontaloscillatory movement to said driving mechanism to bring about the cyclicvertical displacement of said table with respect to said base memberincluding means for reversing the direction of motion of the togglejoints in alternate rows to reduce horizontal forces imparted to saidtable from said driving mechanism as a result of the horizontaloscillatory movement of said drive rods.

References Cited in the le of this patent UNITED STATES PATENTS StuhlerFeb. 13, 1934 Buchanan et al. May 9, 1944 Ongaro Apr. 22, 1958 FOREIGNPATENTS France July 29, 1935 France Dec. 17, 1943 Great Britain Dec. 11,1957

